Neurofibrillary lesions are a hallmark pathology of tauopathic neurodegenerative disorders such as Alzheimer's disease. The lesions are composed primarily of tau, a microtubule-associated protein that normally functions to promote tubulin assembly, microtubule stability, and cytoskeletal integrity. The tau that accumulates in disease lesions differs from microtubule-associated protein in its state of aggregation and posttranslational modification. Despite progress in describing the macroscopic aggregation pathway, a consensus has not emerged on how lesion formation links to events on the molecular level or to the cellular mechanisms of neurodegeneration. To address these crucial questions, this laboratory developed powerful methods for quantifying tau fibrillization in vitro, a first-order kinetic model rationalizing assembly behavior, and tight-binding ligands potentially useful for detecting and inhibiting tau aggregation. On the basis of these findings, it is hypothesized that under near physiological conditions tau fibrillizes via a partially folded intermediate in a nucleation-elongation mechanism, and that the reaction pathway creates novel pharmacophores available for selective binding of small-molecule ligands. It is further postulated that posttranslational modifications, mutations, and exogenous effectors trigger or enhance aggregation by selectively interacting with assembly species. The present proposal has three Specific Aims that test these hypotheses. First, the kinetic pathway through which tau fibrillizes will be determined, culminating in a mathematical simulation of the reaction. Second, the mechanism of action of novel fibrillization inhibitors will be established using quantitative assays and structure-activity relationships, leading to target identification and clarification of the potential for pharmacological intervention in the process. Finally, the molecular mechanisms underlying the effects of posttranslational modifications and disease causing mutations will be determined. Together, these data will clarify the molecular events accompanying fibrillization, and feasibility of antagonizing and even reversing early stage tau filament formation in vivo.